Acinetobacter baumannii complex

Background

Microbiology

• Contains Acinetobacter baumannii, Acinetobacter nosocomialis, and Acinetobacter pittii
• Non-motile, non-fermenting Gram-negative bacillus

Antimicrobial Resistance

• A number of mechanisms
• Carbapenem resistance is usually mediated by acquisition of OXA-type class D carbapenemase
  • Less common mechanisms include acquisition of class B (VIM, IMP, and NDM) carbapenemases, loss of outer membrane CarO protein, and modification of AdeABC efflux pump

Management

• Choice of antibiotic depends on susceptibility testing
• Possible options include:
  • Cefepime, ceftriaxone, and cefotaxime
  • Cefiderocol
  • Carbapenems
  • Tigecycline
  • Colistin and polymyxin B (though Acinetobacter junii has inherent resistance)
• Aminoglycosides may not penetrate well into lungs and brain, so are usually avoided
• Bacteriophages are promising

Carbapenem-Resistant Acinetobacter baumannii

• Infection must be distinguished from colonization of the airway or wound
• Resistance may be mediated by a number of β-lactamases, including OXA-24/40-like carbapenemases, OCA-23-like carbapenemases, and metallo-β-lactamases, and often has sulbactam resistance
• Often have concurrent aminoglycoside-modifying enzymes or 16S rRNA methyltransferases, which confer resistance to aminoglycosides including plazomicin
• Single-agent treatment may be sufficient for mild infections
  • High-dose ampicillin-sulbactam is preferred, at a dose of either 9 g IV q8h infused over 4 hours, or 27 g IV q24h continuous infusion
• Combination treatment with at least two agents that have in vitro activity for most other infections
  • Options include ampicillin-sulbactam (preferred), minocycline, tigecycline, polymyxin B, and cefidercocol
  • Ampicillin-sulbactam may remain effective in non-susceptible isolates when used at high doses
  • Fosfomycin and rifampin are not recommended
  • After clinical improvement, step down to single-agent therapy